1. Field of the Invention
The present invention relates to an improved process for manufacturing L-(-)-carnitine from a starting compound containing an asymmetrical carbon atom having a configuration opposite to that of L-(-)-carnitine. The process of the present invention overcomes the drawbacks of conventional processes which first convert a starting compound into an achiral intermediate, generally crotonobetaine or gamma-butyrobetaine, and then convert the achiral intermediate to L-(-)-carnitine. The process of the present invention uses D-(+)-carnitinamide as starting compound.
2. Discussion of the Background
Carnitine contains a single center of asymmetry and therefore exists as two enantiomers, designated D-(+)-carnitine and L-(-)-carnitine. Of these, only L-(-)-carnitine is found in living organisms, where it functions as a vehicle for transporting fatty acids across mitochondrial membranes. Whilst L-(-)-carnitine is the physiologically-active enantiomer, racemic D,L-carnitine has conventionally been used as a therapeutic agent. It is now recognized, however, that D-(+)-carnitine is a competitive inhibitor of carnitine acyltransferases, and that it diminishes the level of L-(-)-carnitine in myocardium and skeletal muscle.
It is therefore essential that only L-(-)-carnitine be administered to patients undergoing haemodialysis treatment or treatment for cardiac or lipid metabolism disorders. The same requirement applies to the therapeutic utilization of acyl derivatives of carnitine for treating disorders of the cerebral metabolism, peripheral neuropathies, peripheral vascular diseases and the like. These disorders are typically treated with acetyl L-(-)-carnitine and propionyl L-(-)-carnitine, which are obtained by acylating L-(-)-carnitine.
Various chemical procedures have been proposed for the industrial-scale production of carnitine. Unfortunately, these procedures are not stereospecific and produce racemic mixtures of D-(+)- and L-(-)-isomers. It is thus necessary to apply resolution methods in order to separate the enantiomeric constituents of the racemate.
Typically, the D,L-racemic mixture is reacted with an optically active acid (e.g. D-(-)-tartaric acid, D-(+)-camphorsulfonic acid, (+)-dibenzoyl-D-(-)-tartaric acid, N-acetyl-L-(+)-glutamic acid and D-(+)-camphoric acid) to obtain two diastereoisomers which can be separated from each other. In the classic process disclosed in U.S. Pat. No. 4,254,053, D-(+)-camphoric acid is used as the resolution agent of a racemic mixture of D,L-carnitinamide, obtaining D-(+)-carnitinamide as a by-product, and L-(-)-carnitinamide which, by hydrolysis, gives L-(-)-carnitine.
However, these resolution procedures are complex and costly, and in all cases result in the production of equimolar quantities of L-(-)-carnitine and D-(+)-carnitine or a precursor thereof as by-product, having configuration opposite to that of L-(-)-carnitine. Several microbiological processes have recently been proposed for producing L-(-)-carnitine via stereospecific transformation of achiral derivatives obtained from the huge amounts of D-(+)-carnitine (or of a precursor thereof, such as D-(+)-carnitinamide) which are generated as by-products in the industrial production of L-(-)-carnitine.
These processes are generally predicated upon the stereospecific hydration of crotonobetaine to L-(-)-carnitine, and differ principally by virtue of the particular microorganism employed to accomplish the biotransformation of interest. See, for example, the processes disclosed in: EP 0 2 1444 (HAMARI), EP 0 122 794 (AJINOMOTO), EP 0 148 132 (SIGMA-TAU), JP 275689/87 (BIORU), JP 61067494 (SEITETSU), JP 61234794 (SEITETSU), JP 61234788 (SEITETSU), JP 61271996 (SEITETSU), JP 61271995 (SEITETSU), EP 0 410 430 (LONZA), EP 0 195 944 (LONZA), EP 0 158 194 (LONZA), and EP 0 457 735 (SIGMA-TAU).
On the other hand, JP 62044189 (SEITETSU) discloses a process for stereoselectively producing L-(-)-carnitine starting from gamma-butyrobetaine, which is in turn obtained enzymically from crotonobetaine.
All of these processes have several drawbacks. First, D-(+)-carnitine must first be converted to an achiral compound (crotonobetaine, gamma-butyrobetaine) before it can be used as the-starting compound in all of the aforesaid microbiological processes.
In addition, the microbiological procedures proposed to date have not proven practicable for manufacturing L-(-)-carnitine on an industrial scale for one or more of the following reasons:
(i) the yield of L-(-)-carnitine is extremely low; PA1 (ii) the microorganisms must be cultivated in a costly nutritive medium; (iii) the microorganism can only tolerate low concentrations [up to 2-3% (w/v)] of crotonobetaine; PA1 (iv) side reactions occur, such as the reduction of crotonobetaine to gamma-butyrobetaine or the oxidation of L-(-)-carnitine to 3-dehydrocarnitine. PA1 (2) acylation (particularly, mesylation) of 2to 3 can be carried out in the absence of solvents, particularly pyridine the use of which brings about serious drawbacks; PA1 (3) the ester group of acyl derivatives 3is converted into the carboxyl group of acyl derivative 4 via simple acid hydrolysis, thus avoiding the drawbacks of hydrogenation reduction, which are particularly serious when the process is conducted on an industrial scale. PA1 Acyl D-(+)-carnitine 4is isolated by filtering off the catalyst and then lyophilizing or concentrating the aqueous solution. PA1 Acyl D-(+)-carnitine 4is then converted to the lactone 5 of L-(-)-carnitine. PA1 The lactonization is carried out in an aqueous basic environment: either with NaHCO.sub.3 (ratio 1:1) or with an AMBERLITE IRA-402 basic resin activated in HCO.sub.3.sup.- form or with an LA2 resin. The lactone is isolated by evaporating the aqueous solution or precipitating it as a salt (e.g. as tetraphenylborate or reineckate). PA1 Finally, lactone 5 is converted to L-(-)-carnitine inner salt 6. The lactone is dissolved in water and the resulting solution treated with a base such as NaHCO.sub.3 (ratio 1:1), for 8-24 hours. PA1 L-(-)-carnitine is purified from the salts which formed the X.sup.- anion, from the excess--if any--of the acyl halogenide, from pyridine, and the like, by chromatographing the aqueous solution on a strongly acidic resin such as IR 120, eluting with water and then with NH .sub.4 OH, or--alternatively eluting first on a strongly basic resin such as AMBERLITE IRA 402 activated in OH.sub.- form and thereafter on a weakly acid resin such as AMBERLITE IRC-50. PA1 NaHCO.sub.3 (0.46 g;5.4 mmoles) was added to a solution of methanesulfonyl D-(+)-carnitine chloride (1.5 g; 5.4 mmoles) in H.sub.2 O (25mL) and the resulting solution was kept under stirring for 20 hours. The solution was then lyophilized, the residue taken up with CH.sub.3 CN and the undissolved solid filtered off. Following solvent evaporation, 0.98 g, of the title compound were obtained. PA1 Column=Nucleosil 5-SA; diameter=4 mm; length=200 mm PA1 Eluant=CH.sub.3 CN/KH.sub.2 PO.sub.4 50 mM (65/35) pH=3.5 with H.sub.3 PO.sub.4 PA1 Flow-rate=0.75ml/min PA1 Retention time=19.23min PA1 Detector=RI Waters 410 PA1 Column=Nucleosil 5-SA; diameter=4 mm; length=200 mm PA1 Eluant=CH.sub.3 CN/KH.sub.2 PO.sub.4 50 mM (65/35)pH=3.5 with H.sub.3 PO.sub.4 PA1 Flow-rate=0.75 ml/min PA1 Retention time=19.48 min PA1 Detector=RI Waters 410 PA1 column: Nova-pak C.sub.18 (4.mu.) Cartridge PA1 length: 100 mm PA1 diameter: 5.0 mm PA1 Eluant: PA1 Solution A: 5 mM tetrabutylammonium hydroxide (TBA.sup.+ OH.sup.-) and PA1 Solution B: Acetonitrile 75 mL PA1 Detector=Perkin-Elmer Fluorimeter PA1 wherein R has the previously defined meanings, with the resulting formation of a leaving group OR thus obtaining the ester 3 of acyl D-(+)-carnitine; and (c) conversion of 3 to L-(-)-carnitine inner salt 6.
These side reactions reduce the final yield of
L-(-)-carnitine.
In order to overcome all of the aforesaid drawbacks of the known processes, in the Italian patent application RM 92 A 000 915 filed on Dec.21, 1992 in the name of the same applicants as the present application, not available to public inspection at the filing date of this application, a process has been disclosed which allows high yields of L-(-)-carnitine to be obtained starting from a by-product having configuration opposite to that of L-(-)-carnitine (such as D-(+)-carnitinamide) with no need to first convert the starting by-product into an achiral intermediate.
This process which is illustrated in the following reaction scheme 1: ##STR1## comprises hydrolyzing a D-(+)-carnitinamide salt 1 to D-(+)-carnitine 2 and esterifying 2 into ester 3 (via known methods) wherein R1 is preferably arylalkoxy, e.g. benzyloxy.
The ester 3 is then converted to the acyl derivative 4 wherein Y, which can be the same as X, is preferably a counterion, e.g. perchlorate, imparting solubility to 4. OR is a leaving group wherein R is preferably an alkylsulfonyl group having 1-12 carbon atoms, e.g. mesyl.
The acylation of 3 to 4 is carried out preferably in pyridine by reacting the ester 3 with an acylating agent RY wherein Y is halogen and R is an acyl group as defined above. Preferably RY is the chloride of the selected acyl group.
The ester group --COR.sub.1 of 4 (R.sub.1 =benzyloxy) is hydrogenated to carboxyl group thus giving acyl D-(+)-carnitine 5 which is converted to the lactone 6 of L-(-)-carnitine. The lactonization is suitably carried out in an aqueous basic environment: either with NaHCO.sub.3 (ratio 1:1) or with an AMBERLITE IRA-402 basic resin activated in HCO.sub.3 form or with an LA2 resin. The lactone is isolated by evaporating the aqueous solution or precipitating it as a salt (for example, as tetraphenylborate or reineckate).
Finally, lactone 6 is suitably converted to L-(-)-carnitine inner salt 7. The lactone is dissolved in water and the resulting solution treated with a base such as NaHCO.sub.3 (ratio 1:1), for 8-24 hours.
L-(-)-carnitine can suitably be purified from the salts which are formed from the X anion, from the excess, if any, of the acyl halogenide, from pyridine, and the like, by chromatographing the aqueous solution on a strongly acidic resin such as IR 120, eluting with water and then with NH.sub.4 OH, or alternatively eluting first on a strongly basic resin such as AMBERLITE IRA 402 activated in OH form and thereafter on a weakly acid resin such as AMBERLITE IRC-50.
The process of the present invention which is illustrated in the following reaction scheme 2 constitutes a remarkable improvement over the previous process. ##STR2## Indeed: (1) D-(+)-carnitinamide 1 is directly converted to ester 2 (without previous conversion to D-(+)-carnitine);
In detail, with reference to the reaction scheme 2, D-(+)-carnitinamide 1 is converted into ester 2 via conventional procedures, in the presence of an excess of alcohol, preferably an alkanol having 1-4 carbon atoms, by acid catalysis, e.g. with gaseous HCl or concentrated H.sub.2 SO.sub.4.
X.sup.- is for instance a halogenide, (preferably chloride); sulphate; phosphate; perchlorate: metaperiodate; tetraphenylborate; an alkylsulphonate having from 1 carbon atom (methanesulphonate) to 12 carbon atoms (dodecylsulphonate); trifluoroacetate; tetrahalogenborate; fumarate or alkylsulphate having 10-14 carbon atoms.
Suitable esters 2 include those esters wherein R1 is a straight or branched alkyl group having 1-11 carbon atoms, preferably n-butyl or isobutyl.
The ester 2 is then converted to the acyl derivative 3 wherein OR is a leaving group wherein R is an alkylsulfonyl group having 1-12 carbon atoms, formyl or trifluoroacetyl. Preferably, the alkylsulfonyl group is selected from methanesulfonyl (mesyl), p-toluenesulfonyl (tosyl), p-bromobenzenesulfonyl (brosyl), p-nitrobenzenesulfonyl (nosyl), trifluoromethanesulfonyl (triflyl), nonafluoromethanesulfonyl (nonaflyl) and 2,2,2-trifluoroethanesulfonyl (tresyl). Mesyl is particularly preferred.
The acylation of 2 to 3 is carried out by reacting the ester 2 with R.sub.2 O, the anhydride of the selected acid wherein R is an acyl group as defined above.
The acylation reaction is carried out in inert anhydrous solvents, such as methylene chloride or acetonitrile or directly in a molten mixture of the two reactants, without any solvent. The acylating agent is added at ratios ranging from 1:1 to 1:5, preferably 1:3, at temperatures comprised between 40.degree. C. and 80.degree. C., for 8-48 hours.
The compound 3 can be isolated (it is not mandatory to isolate the compound 3, as will be shown below), via precipitation with a suitable solvent, such as ethyl ether or hexane. The compound is then purified via crystallization or by eluting its aqueous solution on a weak basic resin such as AMBERLITE IR 45 (Rohm and Haas) or shaking the aqueous solution with a LA-2-type weak basic resin diluted in hexane, and finally lyophilizing or concentrating the aqueous solution.
The ester group -COOR.sub.1 of 3 converted to the carboxyl of acyl D-(+)-carnitine 4 via acid hydrolysis with conventional procedures.
Conversion of acyl D-(+)-carnitine 4 to lactone 5 and the conversion of this latter compound to L-(-)-carnitine 6 are carried out as disclosed in the previously cited Italian patent application RM92A000915. This disclosure follows, except that the numbers designating the specific compounds in the Italian patent application have been replaced with the corresponding numbers for the same compounds used herein (e.g., the acyl D-(+)-carnitine is designated "5" in the Italian application and "4" herein; compare SCHEME 1 and SCHEME 2, supra.)
Preparation of the lactone of L-(-)-carnitine chloride 5b).
Yield: quantitative
TCL=silica gel Eluant=CHCl.sub.3 /MeOH/iPrOH/H.sub.2 O/AcOH 42/28/7/10.5/10.5
Rf=0.1
.sup.1 H NMR (D.sub.2): .delta.5.33-5.24 (m, 1H, --CHOCO--); 3.96-3.88 (m, 3H, --CH.sub.2 N.sup.+ Me.sub.3, --CHHCOO--); 3.53-3.44 (m, 1H, --CHHCOO--); 3.24 (s, 9H, --N.sup.+ Me.sub.3)
.sup.13 C NMR (D.sup.2 O): .delta.6 172.428; 70.671; 68.094; 56.991; 41.394 IR (KBr)=.div.(cm.sup.-1) 1850 (C=O)
HPCL
Preparation of the lactone of L-(-)-carnitine methanesulfonate (5c).
An aqueous solution of methanesulfonyl D-(+)-carnitine chloride (1.5 g; 5.4 mmoles) was percolated through an IRA-402 resin (30 g) activated to HCO.sub.3 -- form and cooled to 5.degree. C., eluting with water at 5.degree. C. till complete elution (controlled by TCL).
The eluate was kept at room temperature for 4hours.
Following evaporation of the aqueous solution, 1.3 g of a raw product which was taken up with CH.sub.3 CN, were obtained.
Evaporation of the organic solvent yielded 1 g of a white solid.
Yield: 80%.
Differential thermal analysis: incipient decomposition at 160.degree. C. [.alpha.].sub.D.sup.25 +-24.7.degree. (c=1% MeOH) TCL=silica gel Eluant=CHCl.sub.3 /MeOH/iPrOH/H.sub.2 O/AcOH 42/28 /7/10.5/10.5
Rf=0.1
______________________________________ Elementary analysis for C.sub.8 H.sub.17 NO.sub.5 S C % H % N % ______________________________________ Calculated 40.16 7.16 5.85 Found 39.61 7.13 5.77 ______________________________________
.sub.1 H NMR (D.sub.2 O): .delta.5.35-5.25 (m, 1H, --CHOCO--); 3.98-3.89 (m, 3H, --CH.sub.2 N.sup.+ Me.sub.3, --CHCOO--); 3.54-3.46 (m, 1H, --CHHCOO--); 3.26(s, 9H, --N.sup.+ Me.sub.3); 2.81(s, 3H, CH.sub.3 SO.sub.3 --)
.sup.13 C NMR (D.sub.2 O): .delta.172.428; 70.671; 68.094; 56.991; 45.320; 41.394
IR (KBr)=.nu.(cm.sup.-1)1835(C.dbd.O)
HPCL
Preparation of L-carnitine inner salt (6) from the lactone of L-(-)-carnitine methanesulfonate (5 c).
NaHCO.sub.3 (0.34 g; 4 mmoles) was added to a solution of the lactone of L-(-)-carnitine methanesulfonate (0.96 g; 4 mmoles) in H.sub.2 O(20mL) and the resulting solution was kept under stirring at room temperature for 20 hours. The solution was then percolated through AMBERLITE IR-120 resin (20 g) eluting first with water till neutrality to remove methanesulfonic acid and then with 2% NH.sub.3 aqueous solution collecting the eluate till complete elution of L-(-)-carnitine inner salt (controlled by TLC).
Following evaporation of the aqueous solution 0.64 g of L-(-)-carnitine inner salt were obtained.
Alternatively, the reaction mixture was percolated through IRA-402 resin (20 g) activated to OH.sup.31 form, eluting with H.sub.2 O till neutrality. The eluate was then percolated through IRC-50 resin (20 g) till complete elution of L-carnitine inner salt (controlled by TLC). Following evaporation of the aqueous solution, 0.64 g of L-(-)-carnitine inner salt were obtained.
Yield: quantitative.
The enantiomeric excess (e.e.) was assessed via the following HPLC method, after L-(-)-carnitine was derivatized with a chiral reagent. As chiral reagent, (+)-1-(9-fluorenyl) ethyl chloroformate (FLEC) was used.
50 mM KH.sub.2 PO.sub.4 75 mL PA2 Acetonitrile 25 ml PA2 brought to pH 7 with 1N KOH PA2 KH.sub.2 PO.sub.4 5 mM 25 mL PA2 Excitation=260 nm PA2 Slit=20 nm PA2 Emission=315 nm PA2 Slit=5 nm
______________________________________ Elution schedule time % A % B ______________________________________ 0 100 0 15 100 0 16 0 100 22 0 100 23 100 0 30 stop ______________________________________
L-(-)-carnitine had previously been derivatized with FLEC via the following method; 50 .mu.L of L-(-)-carnitine solution (prepared by dissolving 10 mg carnitine in 50 mL of 50 mM TBA.sup.30 OH.sup.- brought to pH 7 with concentrated H.sub.3 PO.sub.4) and 200 .mu.L of solution consisting of 1 mL FLEC in 3 mL acetone, were kept under stirring at 80.degree. C. for 20 minutes.
The solution was cooled and 4 mL of solution A were added thereto. 5 .mu.L of the resulting solution were injected. L-(-)-carnitine K.sup.1 =5.79 D-(+)-carnitine K.sup.1 =4.82, absent ##EQU1## Preparation of L-carnitine inner salt (6) from methanesulfonyl-D-carnitine chloride (4 b ).
NaHCO.sub.3 (0.46 g; 5.4 mmoles) was added to a solution of methanesulfonyl-D-carnitine chloride (1.5 g; 5.4 mmoles) in H.sub.2 O (25 mL) and the resulting solution was kept under stirring at room temperature for 20 hours. Further NaHCO.sub.3 (0.46 g; 5.4 mmoles) was then added and the solution was kept under stirring at room temperature for further 20 hours. The title compound was isolated as previously described for the isolation of 6 from 5 b.
L-carnitine is obtained from methanesulfonyl-D-carnitine through the formation of the lactone 5, as evidenced by NMR, HPLC, IR and TLC analysis carried out on a sample obtained by lyophilizing a portion of the solution 20 hours following first NaHCO.sub.3 addition.
It should be understood that, whereas the process disclosed above has been described, for the sake of clarity, as a sequence of five distinct operating steps, the corresponding industrial process consists of three steps only. When the process of the present invention is carried out as an industrial process, the acyl D-(+)-carnitine ester 3 can be directly converted to L-(-)-carnitine inner salt 6 without isolating either the acyl D-(+)-carnitine 4 or the lactone 5.
In fact, the ester of acyl D-(+)-carnitine 3 is hydrolyzed in an acid environment, then the resulting aqueous solution is concentrated and the concentrate is brought to pH 7-9, preferably 8-9 and kept at this pH value for 30-50 hours yielding L-(-)-carnitine.
In the following example which describes one embodiment of the process of the invention, the intermediate compounds 2, 3 and 4 were isolated so as to exhaustively characterize them from a physico-chemical standpoint.
It will be, however, apparent to any expert in organic synthesis that the industrial process comprises the following steps only: (a) conversion of D-(+)-carnitinamide 1 to the ester of D-(+)-carnitine 2; (b) acylating of the hydroxyl group of ester 2 with an anhydride R.sub.2 O,